Position-specific propensities of amino acids in the β-strand

Background: Despite the importance of beta-strands as main building blocks in proteins, the propensity of amino acid in beta-strands is not well-understood as it has been more difficult to determine experimentally compared to alpha-helices. Recent studies have shown that most of the amino acids have significantly high or low propensity towards both ends of beta-strands. However, a comprehensive analysis of the sequence dependent amino acid propensities at positions between the ends of the beta-strand has not been investigated. Results: The propensities of the amino acids calculated from a large non-redundant database of proteins are found to be highly position-specific and vary continuously throughout the length of the beta-strand. They follow an unexpected characteristic periodic pattern in inner positions with respect to the cap residues in both termini of beta-strands; this periodic nature is markedly different from that of the alpha-helices with respect to the strength and pattern in periodicity. This periodicity is not only different for different amino acids but it also varies considerably for the amino acids belonging to the same physico-chemical group. Average hydrophobicity is also found to be periodic with respect to the positions from both termini of beta-strands. Conclusions: The results contradict the earlier perception of isotropic nature of amino acid propensities in the middle region of beta-strands. These position-specific propensities should be of immense help in understanding the factors responsible for beta-strand design and efficient prediction of beta-strand structure in unknown proteins.